The stability of the rotation of a spacecraft about a desired axis is of concern in many aerospace applications. For example, a transfer orbit spin of a satellite must be stable so that procedures such as attitude determination, thermal control, propellant management, fuel-efficient velocity increment maneuvers, command and telemetry linkage and solar power collection can be accurately performed. When the transfer orbit spin of a satellite is about an intermediate inertia axis, i.e., an axis having a moment of inertia thereabout less than the moment of inertia about a maximum principal axis, and greater than the moment of inertia about a minimum principal axis, the resulting spin is highly unstable. Specifically, a rapidly growing exponential divergence is produced in an uncontrolled intermediate axis spin, as opposed to the slowly-growing divergence which occurs in nutation.
Most geosynchronous communications satellites are of the body-stabilized momentum bias type, and have at least two momentum wheels for providing momentum stabilization on orbit. Such satellites are typically spin-stabilized during transfer orbit, spinning about an axis nearly perpendicular to their momentum wheels. They typically include at least two independent sets of 3-axis gyros to measure body rates to stabilize the satellite during thruster maneuvers during operation.
One solution for obviating the potential for instability is to avoid spinning about an intermediate inertia axis. This can be achieved by imposing constraints in the layout of the satellite in order to produce the desired inertia properties. However, the cost to meet these constraints is excessive as a result of having to produce the desired inertia properties in transfer orbit through deployments to the on-orbit configuration.
Another solution is to employ an active spin axis control system to stabilize the intermediate axis spin. U.S. Pat. No. 4,961,551 to Rosen discloses such a system which uses thrusters under active control with gyro rate sensing. This approach is disadvantageous in that irreplaceable propellant is consumed when using the thrusters, and further, the orbit and momentum of the satellite is disturbed by use of the thrusters.
U.S. Pat. No. 5,012,992 to Salvatore discloses a system for stabilizing intermediate axis spin which uses two momentum wheels and two gimballed momentum wheel platforms in a "vee wheel" configuration. The momentum wheels and platforms are employed to enhance the spin momentum and make the spin axis appear to have the maximum moment of inertia. A difficulty with this system results from deploying the momentum wheel platforms before the end of deployments, and possibly before the end of LAM firing. Typically, the momentum wheel platforms are delicate mission-critical mechanisms which are not designed to take the resulting deployment/LAM loads. Also, since both of the momentum wheels and platforms are utilized, the resulting system is not single fault tolerant.